Glossary

Cytochromes: Specialized enzymes within mitochondria and other cell structures. Different cytochromes play impor­ tant roles in metabolizing toxic substances, drugs, and other chemicals, as well as in producing adenosine triphos­ phate (ATP). Cytokines: A family of molecules, produced primarily by cells of the immune system, that regulate cellular interac­ tions and other functions. Many cytokines play impor­ tant roles in initiating and regulating inflammation. Cytoplasm: The substance filling the cell, including the cytosol as well as mitochondria, endoplasmic reticulum, and other cell structures (organelles) but excluding the nucleus.

Cytokines: A family of molecules, produced primarily by cells of the immune system, that regulate cellular interac tions and other functions. Many cytokines play impor tant roles in initiating and regulating inflammation. Cytoplasm: The substance filling the cell, including the cytosol as well as mitochondria, endoplasmic reticulum, and other cell structures (organelles) but excluding the nucleus. Cytosol: The fluid portion of the cytoplasm. Electron: A subatomic particle with a negative charge. Endocytosis: Mechanism by which specific molecules are ingested into the cell. Endoplasmic reticulum: A system of folded membranes that loop back and forth, spreading throughout the cyto plasm and providing a large surface area for cell reactions. Endothelial cells: Type of cell lining the body cavities and blood vessels; control the passage of materials and the tran sit of white blood cells into and out of the bloodstream. Enzyme: A substance, usually a protein, that directs and accelerates chemical reactions in the body but does not itself undergo permanent change. Expression (i.e., gene expression): The process by which the genetic information encoded in a gene's DNA sequence is converted into a functional protein. Fatty Acids: A major component of fats that is used by the body for energy and tissue development. Fibrosis: The formation of scar tissue. Free radicals: Highly reactive molecular fragments that fre quently contain oxygen. (See reactive oxygen species [ROS].) Genotype: The complete genetic makeup of an organism deter mined by the particular combination of alleles for all genes. Hepatic vein: A large vessel that receives blood after it has passed through the central veins of the liver lobules. Hepatocytes: The principal cells of the liver, which carry out most of the liver's metabolic activities. Heterozygous: Carrying two different alleles of a given gene. Homozygous: Carrying two copies of the same allele of a given gene. Hyperlipidemia: Excess fat in the blood. Hyperuricemia: Excess uric acid in the blood. Hypoxia: Lower-than-normal levels of oxygen. Isozymes/Isoenzymes: Enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Ketosis: Abnormal accumulation in the body of ketones, which are end products of fatty acid metabolism. Ketosis occurs when the body cannot metabolize sufficient carbo hydrates to generate the energy needed (e.g., in patients with diabetes or during starvation). K m : A measurement used to describe the activity of an enzyme.
It describes the concentration of the enzyme's substrate at which the enzyme works at 50 percent capacity.

Glossary
Kupffer cells: Specialized immune cells in the liver that fil ter bacteria and other foreign substances from the blood and produce antibodies and cytokines. (See also sinusoids.) Lactic acidosis: A condition characterized by the accumula tion of lactic acid in bodily tissues. Lipids: Fatty substances, including simple fats, their major components (i.e., fatty acids), and various fat-soluble sub stances (e.g., cholesterol). Lipid peroxidation: The sequential breakdown of fatty sub stances in cells by chemical oxidation, leading eventually to the destruction of membranes within and surrounding the cell. Lobule: A cylindrical structure about 2 millimeters in diam eter that is the basic functional unit of the liver. The liver can be composed of up to 100,000 lobules. Macrophage: A type of immune cell that ingests foreign par ticles and microorganisms and synthesizes proteins and other substances important in inflammatory responses, including cytokines. Macrophages that reside in the liver are called Kupffer cells. Metabolism: The totality of chemical reactions occurring in a cell, an organ, or the body. The term sometimes is applied more narrowly to the breakdown of a particular substance (e.g., alcohol) by specific enzymes. Microsomes: Small vesicles derived from fragmented endo plasmic reticulum produced when tissues such as liver are mechanically broken (homogenized). Microsomes contain the cell's cytochrome P450 (CYP) enzymes, involved in oxidative metabolism. Microsomal ethanol-oxidizing system: An enzyme system involving cytochrome P450 that breaks down alcohol and generates toxic products, such as acetaldehyde and reactive oxygen species (ROS). Microtubules: Any of the minute tubules in cell cytoplasm that are composed of the protein tubulin and form important structural components.

Mitochondria: Structures within cells that generate most of the cells' energy through the production of adenosine triphosphate (ATP).
Mitochondrial electron transport system: see Respiratory chain. NAD/NADH: Nicotinamide adenine dinucleotide (NAD) is a molecule that binds with hydrogen atoms and becomes reduced NAD, or NADH, during alcohol metabolism and other chemical reactions in the cell. NAD and NADH move hydrogen atoms back and forth between various oxidation-reduction reactions, helping to maintain balance between oxidation and reduction in the cell.
Oxidation: A chemical reaction that results in a loss of electrons by a substance and which usually involves removing a hydrogen atom from a molecule or adding oxygen to it, or both. (See reduction.) Oxidative stress: An imbalance between oxidants (e.g., free radicals) and antioxidants that can lead to excessive oxida tion and cell damage.

Perivenous:
Referring to the region of a liver lobule sur rounding the central vein. Peroxisomes: A cytoplasmic cell organelle containing enzymes that act in the production and decomposition of hydrogen peroxide. Phospholipid: A lipid that contains a phosphate group. Polymorphism: Existence of a gene in several allelic forms. Proliferation: The growth and reproduction of cells. Proteins: Molecules composed of chains of amino acids linked together. Proteins help maintain the cell's structure and participate in many biological functions, including the reg ulation of metabolic reactions. The shape and function of a protein is determined by the sequence of its amino acids. Reactive oxygen species (ROS): Highly reactive oxygen-contain ing free radicals that are generated during oxidative metabolism. ROS can react with and damage lipids, proteins, and DNA in cells, causing oxidative stress. Common ROS include hydrogen peroxide, superoxide radicals, and hydroxyl radicals. Receptor: A protein on the surface of a cell that recognizes and binds to chemical messengers. Redox/Redox state: Shorthand for reduction/oxidation reac tions. The term redox state is often used to describe the balance of NAD and NADH in a biological system such as a cell or organ. An abnormal redox state can develop in a variety of deleterious situations. Reduction: The reverse of oxidation, reduction is a chemical reaction that results in a gain of electrons by a substance and which usually involves removing an oxygen atom from a molecule, or adding hydrogen to it, or both. Respiratory chain: The electron transport system located in the mitochondria, in which electrons released by NADH are passed on to a series of other molecules that first accept the electrons and then pass them on to the next molecule in the chain. The electrons ultimately are transferred to oxygen to generate water. These successive reactions provide enough energy to drive the synthesis of adenosine triphosphate (ATP) molecules. Retinol: Vitamin A. Sinusoids: Channels in a liver lobule that bring blood and nutri ents to the hepatocytes, similar to capillaries in other organs. Sinusoids are lined with endothelial cells and Kupffer cells.
Stellate cell: A star-shaped liver cell that serves as the primary storage site for vitamin A compounds and fat molecules; activation of stellate cells plays a central role in the devel opment of fibrosis. Substrate: A substance upon which an enzyme acts. Superoxide: A destructive reactive oxygen species (ROS) produced as a byproduct of some oxidation reactions. Tumor necrosis factor alpha (TNF-α): A type of cytokine that promotes inflammatory responses, stimulates neu trophils and macrophages, induces fever, and induces macrophages to produce cytokines.